排版小内联交换三角形

Question

以下是 Anton Geraschenko 的堆叠笔记。

我该如何排版箭头三角形？我想过用underset某种方法，但是箭头不够长，间距也不起作用，更不用说我不知道​​如何制作小对角箭头了。

Answer 1

堆栈注释？有人提到堆栈了吗？

\documentclass{article}
\usepackage{stackengine,amsmath,graphicx}
\stackMath
\newcommand\triar[3]{%
  \vcenter{\hbox{$%
  #1%
  \setbox0=\hbox{$\rightarrow#3\leftarrow$\kern-4pt}%
  \stackunder[0pt]{\xrightarrow{\kern\wd0}}%
    {%
      \rotatebox[origin=r]{-20}{$\rightarrow$}%
      \raisebox{-2pt}{$#3$}%
      \rotatebox[origin=l]{+20}{$\leftarrow$}%
    }%
  #2%
  $}}%
}
\begin{document}
whose objects are $\mathcal{P}$ morphisms to $Y$, with 
\(
\left\{\triar{X_i}{X}{Y}\right\}
\)\medskip

but not this: 
\(
\left\{\triar{X_i}{X}{Y_\text{long middle element}}\right\}
\)
\end{document}

Answer 2

和tikz-cd：

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-cd}

\newcommand{\tricd}[3]{%
  \begin{tikzcd}[
    baseline={([yshift=-axis_height]mycd)},
    ampersand replacement=\&,
    every matrix/.append style={
      name=mycd,
      nodes={inner sep=0pt, outer sep=2pt}
      },
    row sep=0pt, column sep=12pt
    ]
    #1\ar[rr]\ar[to=mycd-2-2.west]\&\&#2\ar[to=mycd-2-2.east]\\
    \&#3
  \end{tikzcd}%
  }
  
\begin{document}
whose objects are $\mathcal{P}$ morphisms to $Y$, with 
\(
\left\{\tricd{X_i}{X}{Y}\right\}
\)\medskip

but not this: 
\(
\left\{\tricd{X_i}{X}{Y_\text{long middle element}}\right\}
\)
\end{document}

PS = 感谢 Qrrbrbirlbel他们的解决方案垂直对齐问题。